I use a bt sensor that wasn't designed for pools.

https://i.imgur.com/vfjC3uH.jpg

https://www.amazon.com/gp/aw/d/B07R586J37?psc=1&ref=ppx_pop_mob_b_asin_title

I ran it through a food vaccum sealing bag and then put it into this

https://www.amazon.com/gp/aw/d/B079HV3TC9?psc=1&ref=ppx_pop_mob_b_asin_title

Then I weighed it down with a rock so it sits mid way down my pool.

The battery has lasted over 2 years ago far. I have 8 of these sensors and none died yet. I'm also monitoring outdoors, my attic, various rooms and one sensor right in the exit vent of the ac. That way if the ac quits working I'll know sooner than I otherwise would (house is well insulated and it usually takes a day of it not working before I notice).

I use Waterproof Phone pouch with a floaty Key chain and a retractable cable on a carabiner that I usually have hooked on my float vest. It fits my Samsung in an otterbox and I have yet to have any issues with it.